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I?  J  WIKKLI-SS   TI-;i  K<    I;     I'!!  .      \  MI 

I  2  ^s  I         WIRELESS  TELEPHONY 


I  2 
I  6 


5  • 


FROM 

RACTICAL  PHYSICS 


BY 

ROBERT  ANDREWS  MILLIK 

AND 

HENRY  GORDON  GALE 

IN  COLLABORATION  WITH 

WILLARD  R.  PYLE 


GINN  A>i  -1'A.NY 

BOSTON    •    NEW  YORK    •     CH 
AT1.AJ4TA     •     DALLAS  !  KANCBCO 

COPV HI 


This  book  is  DUE  on  the  last  date  stamped  below 


TK 
5742 


Southern  Branch 
of  the 

University  of  California 

Los  Angeles 


M 


.RADI ATK >  X  S 
ELECTRICAL  RADIATIONS 


421 


485.  Proof  that  the  discharge  of  a  Ley  den  jar  is  oscillatory. 
We  found  in  §  408,  p.  346,  that  the  sound  waves  sent  out 
Wy  a  sounding  tuning  fork  will  set  into  vibration  an  adjacent 
'ork,  provided  the  latter  has  the  same  natural  period  as  the 
ler.  Following  is  the  complete  electrical  analogy  of  this 
riment. 

Let  the  inner  aiid  outer  coats  of  a  Leyden  jar  A  (see  Fig.  455)  be 
[ connected  by  u  loop  of  wire  cdef,  the  sliding  crosspiece  de  being  arranged 

so  that  the  length  of  the  loop  may  be  altered  at  will.    Also  let  a  strip 

of  tin  foil  be -brought  over  the  edge  of  this  jar  from  the  inner  coat  to 
[within  about  1  millimeter 
I  of  the  outer  coat  at  C.    Let 
I  the  two  coats  of  an  exactly 

.similar  jar  II  be  connected 
jwith  the  knobs//  and  n'  by 

a  second  similar  wire  loop 

of   fixed    length.     Let  the 

two  jars  be  placed  side  by 
!  .side  with  their  loops  par- 
'allel,  and  let  the  jar  B  be 
;  successively  charged   and  discharged  by  connecting  its  coats   with   a 
I  static  machine  or  an  induction  coil.    At  each  discharge  of  jar  H  through 

the  knobs  //  and  //'  a  spark  will  appear  in  the  other  jar  at  C,  provided 

the  crosspiece  de  is  so  placed  that  the  areas  of  the  two  loops  are  equal. 

When  de  is  slid  along  so  as  to  make  one  loop  considerably  larger  or 

smaller  than  the  other,  the  spark  at  C  will  disappear. 

The  experiment  therefore  demonstrates  that  two  electrical 
circuits,  like  two  tuning  forks,  can  be  tuned  so  as  to  respond  to 
each  other  sympathetically,  and  that  just  as  the  tuning  forks 
will  cease  to  respond  as  soon  as  the  period  of  one  is  slightly 
altered,  so  this  cl<'<-tri<'  rcxona  !><•<•  disappears  when  the  exact 
symmetry  of  the  two  circuits  is  destroyed.  Since,  obviously, 
this  phenomenon  of  resonance  can  occur  only  between  systems 
which  have  natural  periods  of  vibration,  the  experiment  proves 
that  the  discharge  of  a  Leyden  jar  is  a  vibratorv,  that  is,  an 

51652 


.  455.    Sympathetic  electrical  vibrations 


422 


oscillatory,  phenomenon.  As  a  matter  of  fact,  when  such  a 
spark  is  viewed  in  a  rapidly  revolving  mirror,  it  is  actually  found 
to  consist  of  from  ten  to  thirty  flashes  following  each  other  at 
equal  intervals.  Fig.  456  is  a  photograph  of  such  a  spark. 

In  spite  of  these  oscillations  the  whole  discharge  may  be 
made  to  take  place  in  the  incredibly  short  time  of  t  00* 
of  a  second.    This  fact,  coupled 
with  the  extreme  brightness  of 
the  spark,  has  made  possible  the 
surprising   results  of  so-called 

instantaneous  electric-spark  pho- 

7          rr,,          ,  .  ..  FIG.  456.   Oscillations  of  the 

tography.     The  plate   opposite  electric  spark 

page  425  shows  the  passage  of 

a  bullet  through  a  soap  bubble.  The  film  was  rotated  continu- 
ously instead  of  intermittently,  as  in  ordinary  moving-picture 
photography.  The  illuminating  flashes,  5000  per  second,  were 
so  nearly  instantaneous  that  the  outlines  are  not  blurred. 

486.  Electric  waves.  The  experiment  of  §  485  demonstrates 
not  only  that  the  discharge  of  a  Leyden  jar  is  oscillatory  but 
also  that  these  electrical  oscillations  set  up  in  the  surrounding 
medium  disturbances,  or  waves  of  some  sort,  which  travel  to  a 
neighboring  circuit  and  act  upon  it  precisely  as  the  air  waves 
acted  on  the  second  tuning  fork  in  the  sound  experiment. 
Whether  these  are  waves  in  the  air,  like  sound  waves,  or  dis- 
turbances in  the  ether,  like  light  waves,  can  be  determined  by 
measuring  their  velocity  of  propagation.  The  first  determina- 
tion of  this  velocity  was  made  by  Heinrich  Hertz  (see  oppo- 
site p.  102)  in  1888.  He  found  it  to  be  precisely  the  same  as 
that  of  light,  that  is,  300,000  kilometers  per  second.  This 
result  shows,  therefore,  that  electrical  oscillations  set  up  waves  in 
the  ether.  These  waves  are  now  known  as  Hertzian  waves. 

The  length  of  the  waves  emitted  by  the  oscillatory  spark 

of  instantaneous  photography  is  evidently  very  great,  namely, 

.  about  Vo^ooV.ooV  =  30  meters,  since  the  velocity  of  light  is 


ELECTRICAL  RADIATIONS  423 

300,000,000  meters  per  second,  and  since  there  are  10,000,000 
oscillations  per  second  ;  for  we  have  seen  in  §  382,  p.  323, 
that  wave  length  is  equal  to  velocity  divided  by  the  number 
of  oscillations  per  second.  By  diminishing  the  size  of  the  jar 
and  the  length  of  the  circuit  the  length  of  the  waves  may  be 
greatly  reduced.  By  causing  the  electrical  discharges  to  take 
place  between  two  balls  only  a  fraction  of  a  millimeter  in 
diameter,  instead  of  between  the  coats  of  a  condenser,  elec- 
trical waves  have  been  obtained  as  short  as  ,3  centimeter,  — 
only  ten  times  as  long  as  the  longest  measured  heat  waves. 

487.  Detection  of  electric  waves.  In  the  experiment  of  §  485 
we  detected  the  presence  of  the  electric  waves  by  means  of  a 
small  spark  gap  C  in  a  circuit  almost  identical  with  that  in 
which  the  oscillations  were  set  up.    The  visible  spark  may  be 
employed  for  the  detection  of  waves  many  feet  away  from 
the  source,  but  for  detecting  the  feeble  waves  which  come  in 
from  a  source  hundreds  or  thousands  of  miles  away  we  must 
depend  upon  sounds  produced  in  an  extremely  sensitive  tele- 
phone receiver,  as  explained  in  the  next  section. 

488.  Wireless  telegraphy.   Commercial  wireless  telegraphy 
was  realized  in  1896  by  Marconi  (see  opposite  p.  316),  eight 
years  after  the  discovery  of  Hertzian  waves.    The  essential 
elements  of  a  tuned  wave-train,  or  "  spark,"  system  of  wireless 
telegraphy  are  as  follows: 

The  key  K  at  the  transmitting  station  (Fig.  457,  (1)  )  is  depressed 
to  allow  a  current  from  the  alternator  .4  to  pass  through  the  primary 
coil  P  of  a  transformer  7\,  the  frequency  of  the  alternations  in  practice 
being  usually  about  500  cycles  per  second.  The  high-voltage  current 
induced  in  the  secondary  5  charges  the  condenser  Cl  until  its  potential 
rises  high  enough  to  cause  a  spark  discharge  to  take  place  across  the 
gap  s.  This  discharge  of  Cl  is  oscillatory  (§  485),  and  the  oscillations 
thus  produced  in  the  condenser  circuit  containing  C,,  3,  and  Ll  may,  in 
a  low-power  short-wave  transmitting  set,  have  a  frequency  as  high  as 
1,000,000  per  second.  An  oscillation  frequency  much  lower  than  this 
is  generally  used  and  is  subject  to  the  control  of  the  operator  through 


424 


INVISIBLE  RADIATIONS 


the  sliding  contact  c,  precisely  as  in  the  case  illustrated  in  Fig.  455. 
The  oscillations  in  the  condenser  circuit  induce  oscillations  in  the  aerial- 
wire  system,  which  is  tuned  to  resonance  with  it  through  the  sliding 
contact  c'. 


U) 


Aerial 
Wires 


Transmitting  Station 


a 


jrth 

FIG.  457.   Transmitting  and  receiving  stations  for  wireless  telegraphy 


As  long  as  the  key  7v  is  kept  closed  (assuming  a  500-cycle  alternator 
to  be  used),  1000  sparks  per  second  occur  at  s,  and  therefore  a  regular 
series  of  1000  wave  trains  (Fig.  458)  pass  off  from  the  aerial  every 
second  and  move  away  with  the  velocity  of  light.  If  the  oscillations 
which  produce  a  wave  train  have  a  frequency 
of,  say,  500,000  per  second,  each  wave  in  the 

,  300,000,000 
wave  train  has  a  length  or 


Direction  of 
Propagation 


500,000     ' 

600  meters ;  and  if  these  wave  trains  are 
produced  at  the  rate  of  1000  per  second, 
they  follow  each  other  at  regular  distances 
of  300,000  meters,  that  is,  nearly  200  miles. 
The  waves  sent  out  by  the  aerial  system 
of  the  transmitting  station  induce  like  os- 
cillations in  the  distant  aerial  system  of  the 
receiving  station  (Fig.  457,  (2)  ),  which  is 

tuned  to  resonance  with  it.  In  case  the  receiving  aerial  must  be  tuned 
to  respond  to  very  long  waves,  the  switch  O  is  closed  to  cut  out  the 
condenser  C2,  and  the  inductance,  or  loading  coil,  Bl  is  used ;  whereas, 
to  tune  to  very  short  waves,  the  switch  O  is  opened  and  the  variable 


FIG.  458.   One  wave  train 
from  oscillatory  discharge 


One  of  the  most  notable  developments  of  the  war  was  the  directing  of  a  squadron 
of  airplanes  in  intricate  maneuvers  by  wireless  telephone  either  from  the  ground 
or  by  the  commander  in  the  leading  plane.  The  upper  panel  shows  the  pilot  and 
the  observer  conversing  with  special  apparatus  designed  to  eliminate  plane  noises, 
'  and  the  lower  panel  shows  President  Wilson  talking  by  wireless  to  airplanes 


"\ 

• 


CINEMATOGRAPH  FILM  OF  A  Bui 


iD    THROUGH    A    SOAP    BUBBLE 


The  flight  of  the  missile  may  be  followed  easily.   It  will  be  seen  that  the  bubble 

breaks,  not  when  the  bullet  enters,  but  when  it  emerges.  (From  "  Moving  Pictures,' 

by  F.  A.  Talbot.  Courtesy  of  J.  B.  Lippincott  Company) 


ELECTRICAL  RADIATIONS  425 

condenser  C2  is  brought  into  use,  the  loading  coil  not  being  utilized.1 
The  oscillations  in  the  aerial  circuit  of  the  receiving  station  induce 
exactly  similar  ones  in  the  detector  circuit,  which  is  tuned  to  resonance 
with  the  receiving  aerial  by  means  of  L2,  Ev  and  C3.  The  so-called 
detector  of  these  oscillations  may  be  simply  a  crystal  of  galena  D  in 
series  with  the  telephone  receivers  R.  This  crystal,  like  the  tungar 
rectifier  of  §  374,  has  the  property  of  transmitting  a  current  in  one 
direction  only.2  Were  it  not  for  this  property  the  telephone  could  not 
be  used  as  a  detector,  because  its  diaphragm  cannot  vibrate  with  a  fre- 
quency of  the  order  of  a  million;  and  even  if  it  could,  it  would  produce 
sound  waves  far  above  the  limit  of 
hearing.  Because  of  this  rectifying 
property  of  the  crystal  the  receiver 
diaphragm  is  drawn  in  only  once 
while  the  oscillations  produced  by  a 
given  wave  train  last,  this  effect  being 
due  to  the  rectified  pulsating  current 
which  passes  in  one  direction  through 
the  receivers  and  then  ceases  until  the 
oscillations  due  to  the  next  spark  ar-  FlG  459  United  Stateg  navy 
rive.  Since  1000  of  the  intermittent  standard  radio  receivers 

wave  trains  strike  upon  the  aerial  each 

second,  the  operator  at  the  receiving  station  hears  a  continuous  musical 
note  of  this  pitch  as  long  as  the  key  K  is  depressed.  The  working  of  the  key, 
however,  as  in  ordinary  telegraphy,  breaks  the  regular  series  of  wave 
trains  into  groups  of  wave  trains,  so  that  the  short  and  long  notes  heard  in 
the  receivers  (Fig.  459)  correspond  to  the  dots  and  dashes  of  telegraphy. 
The  receiving  circuit,  when  tuned  as  shown  in  Fig.  457,  (2),  is  highly 
selective ;  that  is,  it  will  not  pick  up  waves  of  other  periods.  The  loading 
coils  Bl  and  B2,  as  well  as  the  two  variable  condensers  C2  and  C8,  are 
usually  omitted  from  small  amateur  receiving  sets;  but  when  this  is 
done,  the  receiving  set  is  less  selective  and  less  sensitive.  The  resist- 
ance of  the  receivers  is  so  high,  usually  from  1000  to  4000  ohms,  that 

1  In  the  diagram  an  arrow  drawn  diagonally  across  a  condenser  indicates 
that,  for  the  sake  of  tuning,  the  condenser  is  made  adjustable.  Similarly,  an 
arrow  across  two  circuits  coupled  inductively,  like  the  primary  and  secondary 
of  the  "oscillation  transformer"  Tn,  indicates  that  the  amount  of  interaction 
of  the  two  circuits  can  be  varied,  as,  for  example,  by  sliding  one  coil  a  longer 
or  shorter  distance  inside  the  other. 

2  Crystal  detectors  have  been  largely  superseded  by  the  "audion  "  for  both 
wireless  telegraphy  and  wireless  telephony. 


426  INVISIBLE  RADIATIONS 

they  do  not  interfere  with  the  oscillations  of  the  condenser  system 
across  which  they  are  placed.  The  receiving  station  shown  in  Fig.  457,  (£), 
may  also  be  used  for  receiving  wireless-telephone  messages.  The  simplified 
circuit  of  an  audion  receiving  station  is  shown  opposite  page  441. 

Although  the  spark,  or  wave-train,  system  of  wireless  teleg- 
raphy is  still  widely  used,  the  "  continuous  wave  "  system  is 
rapidly  displacing  it.  Just  as  sound  waves  differing  slightly 
in  frequency  combine  to  produce  the  phenomenon  of  beats 
(§  396),  so  electrical  oscillations  differing  in  frequency  give, 
when  combined,  a  "  beat  effect."  For  instance,  if  electrical 
oscillations  of,  say,  30,000  per  second  and  31,000  per  second 
combine,  beats  will  occur  at  the  rate  of  1000  per  second, 
which  is  a  frequency  within  the  limit  of  hearing.  The  elec- 
trical oscillations  mentioned  above  have  a  frequency  beyond 
the  limit  of  hearing  and  hence  are  said  to  have^  radio  fre- 
quency ;  but  the  beats  being  within  the  range  of  hearing 
have  an  audio  frequency.  Now  let  us  assume  that  there  is 
at  the  transmitting  station  an  alternating-current  generator 
which  throws  into  the  aerial  powerful  undamped  oscillations 
of  30,000  per  second;  and  suppose  further  that  at  the  receiv- 
ing station  there  is  an  oscillation  generator  which  maintains 
relatively  weak  oscillations  of  31,000  per  second  in  the  local 
receiving  aerial.  These  weak  oscillations  produced  in  the 
receiving  aerial  by  the  local  generator  make  no  sound  in  the 
receiver,  being  above  the  limit  of  hearing ;  but  whenever,  and 
as  long  as,  the  operator  at  the  transmitting  station  depresses 
his  key,  waves  come  in  at  the  rate  of  30,000  per  second, 
strike  against  the  receiving  aerial  and  develop  therein  weak 
oscillations  which  combine  with  those  already  present  to  make 
1000  beats  per  second.  These  beat  effects  are  rectified  by  a 
crystal  or  by  a  vacuum  tube  and  passed  through  the  receiver. 
The  listener,  therefore,  hears  long  and  short  musical  sounds 
just  as  he  does  when  receiving  by  the  spark  system.  The 
beat  method  of  receiving  is  called  the  heterodyne  system. 


ELECTKICAL  KADIATIONS  427 

489.  Modulated  continuous  waves.*  The  vibrations  consti- 
tuting articulate  speech  are  exceedingly  complex,  as  may  be 
seen  from  an  inspection  of  the  full-page  halftone  opposite 
page  346.  Because  of  this  complexity  it  is  impossible  to  trans- 
mit speech  by  means  of  discontinuous  waves  (Fig.  460)  such 
as  are  employed  in  the  system  of  spark  telegraphy  described 
in  the  preceding  section.  The  parts  of  the  voice  lost  because 

Direction  of  propagation • » 

-4 4      -4      -4      4      4      4 — -4- 

FIG.  460.    A  series  of  wave  trains 

of  the  gaps  between  the  wave  trains  would  render  the  language 
unintelligible.  Theoretically  the  voice  could  be  transmitted 
by  continuous  electromagnetic  waves  having  the  frequencies 
of  voice  vibrations,  but  such  a  method  is  entirely  impracti- 
cable on  account  of  the  enormous  length  of  aerial  needed 
to  produce  such  long  waves  and  the  tremendous  amount  of 
power  which  would  be  required.  Therefore,  the  only  satis- 
factory method  thus  far  developed  is  to  transmit  speech 


FIG.  461.    Continuous,  or  carrier,  waves  of  radio  frequency 

on    continuous,    or    "  carrier,"    waves  (Fig.  461)    having    a 
frequency  (radio  frequency)  above  the  limit  of  hearing. 

At  the  sending  station  the  continuous  waves  (Fig.  461) 
are  "  modulated "  by  the  voice  at  the  transmitter ;  that  is, 
the  sound  waves  of  the  voice  act  upon  the  apparatus  in  such 
a  way  as  to  alter  the  otherwise  uniform  amplitude  of  the  series 
of  continuous  waves  (Fig.  462).  These  "modulated"  con- 
tinuous waves  on  reaching  the  aerial  of  the  receiving  station 
produce  corresponding  oscillatory  currents  in  the  wires  of  the 

*  The  pupil  should  master  §§  374,  375,  376,  485,  486,  487,  and  488  before 

reading  the  six  sections  following. 


428 


INVISIBLE  RADIATIONS 


aerial.  By  means  of  a  crystal  or  a  vacuum  tube,  the  oscilla- 
tory currents  are  rectified  into  a  series  of  unidirectional  elec- 
trical currents,  or  pulses,  somewhat  after  the  manner  indicated 


FIG.  462.    Modulated  radio-frequency  waves 

in  Fig.  463.  These  variable  pulses  of  radio  frequency,  on 
reaching  the  telephone  receivers  of  the  listener,  produce  dia- 
phragm vibrations  of  low  frequencies  (audio  frequencies),  which 


FIG.  463.    Rectified  oscillations 

rarely  go  outside  the  limits  of  100  and  3000  vibrations  per 
second.  They  are  represented  by  the  irregular  line  in  Fig.  464. 
The  vibrations  of  the  diaphragms  of  the  receivers,  therefore, 


FIG.  464.   Audio-frequency  variations 

correspond  to  the  vibrations  of  the  voice  of  the  speaker  at 
the  distant  transmitting  station. 

490.  Method  of  producing  continuous  waves.  One  of  the 
most  important  of  the  different  means  of  producing  high- 
power  continuous  waves  is  by  use  of  the  Alexanderson  high- 
frequency  alternator  (see  on  opposite  page).  This  is  an 
alternating-current  dynamo  made  in  various  powers  up  to 
200  kilowatts  (=  268  horse  power),  the  rotor  in  some  of  the 
machines  having  the  very  high  speed  of  20,000  revolutions 
per  minute.  For  transoceanic  telegraphy  these  machines  cause 
currents  of  from  600  to  1200  amperes  to  oscillate  in  the  sending 
aerial.  This  powerful  sustained  oscillation  of  electrons  in  an 
aerial  produces  continuous  electromagnetic  waves  (Fig.  461). 


ij   H  i 


ELECTRICAL  KADIATIONS  429 

491.  The  vacuum  tube.  There  are  several  devices  by  which 
the  voice  waves  may  modulate,  or  vary  the  amplitude  of,  the 
carrier  waves,  the  most  important  being  the  highly  exhausted 
"  vacuum  tube  "  (see  Fig.  465,  the  halftone  opposite  p.  441, 
and  the  drawing  and  legend  opposite  p.  33). 

In  attempting  to  reach  an  understanding  of  an  "  audioii " 
amplifier  or  other  form  of  vacuum  tube,  it  is  well  to  remember 


-Filament  (in  center) 

Grid  (surrounding  filament) 

te  (surrounding  grid  — 
front  half  cut  away) 


Terminals 


FIG.  465.    A  popular  form  of  vacuum  tube  used  in  radio  receiving 

that  a  current  of  electricity  is  a  stream  of  negative  elec- 
trons which,  when  passing  through  a  vacuum,  move  with 
enormous  velocity  (thousands  of  miles  per  second  (§  498)), 
but  when  passing  along  a  wire  (ordinary  conduction)  move 
quite  slowly  (a  few  centimeters  per  second).  Now  we  found 
in  studying  the  tungar  rectifier  (§  374)  that  these  negative 
electrons  escape  freely  from  an  incandescent  filament  under 
certain  conditions.  When  the  battery  B  (Fig.  466)  has  its 
-I-  terminal  connected  to  the  plate  P  of  the  vacuum  tube  and 


430 


INVISIBLE  RADIATIONS 


FIG.  466.    A  two-electrode 
vacuum  valve 


its  —  terminal  to  the  filament  F,  no  current  can  flow  across 
the  vacuum  so  long  as  the  filament  is  cold.  When,  however,  the 
filament  is  maintained  at  incandescence  by  a  battery  A,  the 
negative  electrons  escape  from  it  and  are  drawn  in.  a  steady 
stream  across  the  vacuum  by  the 
attraction  of  the  +  plate  P.  This 
flow  of  —  electrons  from  filament  to 
plate  constitutes  what  is  considered 
by  convention  to  be  a  current  of 
electricity  flowing  the  opposite  way, 
namely,  from  plate  to  filament.  We 
now  see  how  battery  A,  by  keeping 
the  filament  in  a  state  of  incandes- 
cence, merely  establishes  and  main- 
tains one  of  the  conditions  under  which  battery  B  may  discharge 
a  steady  current  through  the  vacuum.  No  electronic  flow 
from  the  cold  plate  to  the  filament  is  ever  possible,  because 
cold  bodies  do  not,  except  in  rare  instances  (see  pp.  441  ff.) 
eject  electrons  from  themselves.  The  vacuum  tube  can 
therefore  be  utilized  as  a 
vacuum  valve,  or  rectifier,  for 
evidently,  if  a  source  of  alter- 
nating current  be  substituted 
for  the  direct  current  source 
(battery  5),  the  vacuum  valve 
would  transmit  current  in  one 
direction  only,  half  of  each 
cycle  being  held  in  check. 

If  a  screen  of  fine  wire  G, 
known  as  a  "  grid,"  be  introduced  between  the  filament  and 
the  plate  of  Fig.  466  (see  Fig.  467)  and  the  grid  be  main- 
tained at  a  sufficiently  high  —  potential  by  a  battery  C,  the 
-  electrons  are  repelled  back  into  the  incandescent  filament 
and  cannot  escape  from  it,  and  thus  the  electronic  flow  is 


FIG.  467.    A  three-electrode  vacuum 
tube 


ELECTRICAL  RADIATIONS 


431 


Condenser 


The  lamp  does  not  burn 


completely  checked;  that  is,  no  current  flows  across  the  vacuum. 
If  now  the  —  potential  of  the  grid  be  varied,  say,  from  zero 
to  the  amount  required  to  stop  the  electronic  flow,  the  current 
from  battery  B  through  the  vacuum  is  thereby  varied  from 
the  possible  maximum  in  Fig.  466  to  zero.  Variation  of  the 
grid  potential,  therefore,  affords  us  a  means  of  controlling 
and  of  varying  the  flow 
of  current  through  a 
vacuum  tube.  Indeed, 
it  is  found  that  sliylit  D.  C.  generator^ 
changes  in  the  grid  volt- 
age produce  surpris- 
ingly great  changes  in 
the  current  through  the 
tube ;  that  is,  the  tube 
is  an  amplifier. 

492.  Transfer  of  energy 
through  a  condenser.  In 
Fig.468,(l),theE.M.F. 
of  the  direct-current  dy- 
namo causes  a  rush  of  . 

,  .  ,  The  lamp  burns 

electrons  out  of  one  side 

of  the  condenser  while 
electrons  to  an  equal 
extent  rush  into  the  other  side.  The  sides  of  the  condenser 
are  thus  charged  +  and  —  and  they  remain  so  as  long  as  the 
dynamo  runs.  It  is  evident  that  under  these  conditions  there 
is  no  flow  of  current  and  that  consequently  the  lamp  does 
not  burn.  If,  however,  an  alternating-current  dynamo  is  used 
(Fig.  468,  (2)),  the  alternating  E.M.F.  causes  an  alternat- 
ing rush  of  electrons  which  charges  the  condenser  first  one 
way  and  then  the  opposite  way.  It  is  clear,  then,  that  with 
an  alternating-current  dynamo,  lamp,  and  condenser  thus 
arranged  we  may  have  an  alternating  current  through  the 


A.  C.  generator 


Condenser 


FIG.  468.    Energy  transferred  through  a 
condenser 


48-2 


INVISIBLE  RADIATIONS 


lamp  which  will  cause  it  to  light  up.  Condensers  of  variable 
capacity  are  widely  used  in  the  circuits  of  wireless  apparatus 
as  aids  in  tuning,  and  they  permit  passage  of  electrical  energy 
in  the  manner  explained  above. 

493.  The  receiving  station.  Fig.  469  represents  a  "regenera- 
tive" receiving  circuit  capable  of  receiving  long  or  short 
waves.  When  the  modulated  waves  (Fig.  462)  reach  the 
tuned  aeriaj  of  the  receiving  station,  they  develop  therein 
feeble  electrical  oscillations  which  induce  oscillations  in  La 
of  the  tuned  grid  circuit.  This  varies  the  potential  of  the 


/Aerial 


Fi<;.  469.    A  regenerative  receiving  circuit 

grid  6?2,  thus  causing  corresponding  changes  in  the  strength 
of  the  electronic  current  flowing  from  the  incandescent  fila- 
ment F2  to  the  plate  P2  and  thence  back  through  the  plate 
coil  PC.  The  plate  circuit  is  so  tuned  with  respect  to  the 
grid  circuit  that  these  current  variations  in  the  plate  coil 
react  inductively  on  the  coil  L2  connected  with  the  grid  cir- 
cuit to  strengthen  the  original  grid-circuit  current.  This 
intensifies  the  variations  in  potential  at  the  grid,  which  in 
turn  intensifies  the  variations  in  strength  of  the  electronic 
current  from  filament  to  plate,  and  this  still  further  intensi- 
fies the  variations  in  potential  at  the  grid,  and  so  on,  up  to 


KLKCTUICAL    RADIATIONS 


483 


the  limit  of  the  electron  supply  in  the  tube.  This  is  thr 
Armstrong  r<<<i<>)icr<ilin'  )>rhi<-ipl^  by  which  very  feeble  oscilla- 
tions produced  by  the  incoming  waves  may  be  amplified  and 
then,  used  to  intensify  the  original  oscillations.  The  energy 
for  regeneration  comes  from  the  battery  B .  When  the  tube  is 
in  use  the  grid  tends  to  accumulate  a  negative  charge  which, 
as  we  have  seen  (§  491),  would  tend  to  block  completely 
the  action  of  the  tube.  Therefore,  a  high-resistance  grid  leak 
r  is  shunted  around  the  condenser  ('  to  permit  the  return 


Variometer 


FIG.  470.    A  two-variometer  tuned-plate-circuit  for  receiving  short  waves 

of  such  a  detrimental  accumulation  of  electrons  to  the  fila- 
ment F^  by  way  of  r  and  LZ.  The  telephone  receivers  used 
in  wireless  work  contain  thousands  of  turns  of  very  fine  wire 
wound  upon  iron  and  because  of  the  consequent  "choke- 
coil  "  effect,  or  impedance,  of  these  coils  for  high-frequency 
changes  in  current  strength,  the  racfoo-frequency  variations 
(Fig.  463)  of  the  plate  current  pass  largely  by  way  of  the 
variable  condenser  (78,  while  the  slower  awdfo-frequency  varia- 
tions (Fig.  464)  of  the  plate  current  pass  readily  through 
the  receivers  to  actuate  the  diaphragm. 

Fig.  470  shows  a  two- variometer  circuit  for  the  reception 
of  short  waves.    A  variometer  is  a  variable  inductance  used 


434  INVISIBLE  RADIATIONS 

for  tuning  and  it  consists  of  two  coils  in  series,  one  of  which 
revolves  within  the  other.  If  current  is  passed  through  the 
variometer  when  the  inner  coil  is  turned  so  that  its  magnetic 
field  combines  with  that  of  the  other  coil  to  make  the  greatest 
resultant  magnetic  field,  the  inductance  of  the  variometer  is 
found  to  have  its  greatest  value  and  the  adjustment  is  then 
for  the  longer  waves,  or  slower  oscillations.  If  the  inner  coil 
is  now  turned  through  180°,  the  resultant  magnetic  field  is 
at  minimum  strength ;  and,  because  of  the  small  inductance, 
the  variometer  is  adjusted  to  the  shorter  waves.  Intermediate 
positions  of  the  inner  coil  are  used  for  wave  lengths  lying 
between-  these  limits.  Complete  tuning  is  accomplished  by 
use  of  the  two  variometers,  the  two  variable  condensers  and 
the  sliding  contact  on  the  aerial  coil. 

494.  The  transmitting  station.  The  vacuum  tube  may  be 
used  not  only  as  a  rectifier,  a  detector,  a  modulator,  and  an 
amplifier,  but  under  certain  conditions  as  a  generator  of  oscil- 
lations varying  over  an  extremely  wide  range  of  frequency  — 
from  less  than  1  oscillation  per  second  to  300,000,000  or 
more  per  second.  Nearly  all  present-day  "  broadcasting "  is 
done  by  use  of  vacuum-tube  generators.  For  high-power 
long-distance  transmission  banks  of  vacuum-tube  amplifiers 
may  be  used  to  throw  into  an  aerial  an  aggregate  power  of 
many  hundreds  of  kilowatts.  Indeed,  at  the  present  time 
rapid  progress  is  being  made  in  the  experimental  construc- 
tion of  power  tubes  each  one  of  which  is  capable  of  giving  an 
amazing  output.  The  life  of  a  vacuum  tube  is  generally  from 
1000  to  5000  hours,  whereas  a  high-frequency  alternator,  such 
as  the  Alexanderson,  will  last  for  many  years. 

It  is  entirely  beyond  the  scope  of  this  book  to  explain  the 
actual  details  of  a  wireless-telephone  transmitting  station. 
However,  the  method  used  at  present  in  high-power  long- 
distance transmission  is  indicated  in  Fig.  471  and  may  be 
outlined  as  follows:  Air  vibrations  produced  by  the  voice 


ELE<  TKK.'AL   RAD1ATK  >.\  S 


485 


make  variations  in  the  current  of  the  primary  circuit  of  the 
telephone  transmitter  (§  376).  This  induces  corresponding 
E.  M.  F.'s  in  the  secondary  circuit,  which  impresses  audio- 
frequency variations  of  potential  upon  the  grid  of  a  vacuum- 
tube  modulator.  The  resulting  changes  of  audio  frequency 
in  the  current  of  the  plate  circuit  of  the  modulator  corre- 
spondingly affect  the  output  of  the  high-frequency  oscil- 
lation generator.  This  modulated  radio-frequency  output  is 


Three-electrode  vacuum-tube 
high-frequency  oscillation  generator 


\Aerial 


(^illation 
Transformer 


Telephone 
Transmit  te 


Vacuum-tube  modulator 
FII;.  471.    High-power  long-distance  wireless-telephone  transmitting  station 

amplified  by  a  bank  of  three-electrode  power  tubes  and  is 
then  delivered  to  the  aerial  through  an  oscillation  trans 
former.  In  broadcasting  stations  (see  opposite  p.  429)  a 
weaker  and  somewhat  simpler  arrangement  of  tubes  is  used. 

XOTK.  The  following  reference  books  will  prove  helpful  to  teachers  and 
to  those  pupils  who  desire  a  more  complete  understanding  of  "wireless  "  : 
(1)  BUCIIER,  Practical  Wireless  Telegraphy,  Wireless  Press,  326  Broad- 
way, New  York  City  ;  (2)  GOLDSMITH,  Radio  Telephony,  Wireless  Press. 
326  Broadway,  New  York  City;  (3)  H.USMANN  and  others,  Radio  Pimm- 
Receiving,  Van  Nostrand  Co.,  8  Warren  St.,  New  York  City;  (4)  M«n:i  - 
CKOKT,  Principles  of  Radio  Communication,  John  Wiley  and  Sons,  432 
Fourth  Ave.,  New  York  City  ;  (5)  SCOTT-TAGT.AKT,  Thermionic  Tubes  in 
Radio  Telegraphy  and  Telephony,  Wireless  Press.  326  Broadway,  New 
York  City ;  (6)  Elementary  Principles  of  Radio  Telegraphy  and  Teleph- 
ony (Radio  Communication  Pamphlet  1),  79  pages,  illustrated,  10  cents, 
Superintendent  of  Documents,  Government  Printing  Office,  Washington, 
D.C.,  1922. 


436 


INVlSUiLK    RADIATIONS 


Although  transoceanic  telephonic  communication  has  been  su< 
cessf'ully  and  repeatedly  accomplished  (see  opposite  p.  441). 
no   regular    service    for    such   communication    has    yet  be 
established. 

495.  The  electromagnetic  theory  of  light.     The   stud 
electromagnetic  radiations,   like  those  discussed    in  the  pre- 
ceding paragraphs,  has  sho\vn  not  only   that  they  have   the! 
speed   of    light    but   that   they   are    reflected,    refracted,   and! 
polarized,  —  in  fact,  that  they  possess  all  the  properties  of  light 
waves,   the  only  apparent  difference   being   in    their  greater 
wave  length.    Hence  modern  physics  /•<'</«>•</•*  li'jlit  <ix  <tn  i-/i',-f,-<>- • 
magnetic  phenomenon;  that  is,  light  waves  are  thought  to  be; 
generated  by  the  oscillations  of  the  electrically  charged  parts' 
of  the  atoms.     It  was  as  long  ago  as  1864  that  Clerk-Maxwell,  i 
(see  opposite  p.  102),  of   Cambridge,  England,  one  of  the 
world's  most  brilliant  physicists  and  mathematicians,  showed  : 
that  it  ought  to  be  possible  to  create  ether  waves  by  means 
of  electrical  disturbances.    But  the  experimental  confirmation 
of  his  theorv  did  not  come  until  the  time  of   Ilert/.'s  experi- 
ments (1SSS).    Maxwell  and  Hertz  together,  therefore,  share 
the  honor  of  establishing  the  modern  electromagnetic  theory 
of  light. 

('ATHOI)K    AM)    IvONTGKN     It  AYS 

496.  The  electric  spark  in  partial  vacua.    Let  n  and  //  ( I  i- .  I  TIM  ! 

be  the  terminals  of  an  induction  coil  or  static  machine  :  <-  andjfj  electrode-,  j 
sealed  into  a  glass  tube  lid  or  80 
centimeters  Ion-:  //.  a  rubber 
till"-  leading  to  an  air  ]>uni]>  by 
which  the  tube  may  be  ex- 
hausted. Let  the  roil  be  started 
ln-t'ore  the  exhaustion  is  begun. 
A  spark  will  pass  between  a  and 
//,  since  nh  is  a  very  much  shorter 


Fn..  47^.    Discharge  in  partial  vacua 


path  than  rf.    Then  let  the  tube  be  rapidly  exhausted.    When  the  pres- 
sure has  been  reduced   to  a  few   centimeters  of  mercury,  the  discharge 


51652 


